The term "MCrAlY" is a shorthand way of referring to well known temperature and oxidation/corrosion resistant alloy systems comprising in general one or more of nickel, cobalt and iron as the major "M" constituent, together with chromium and aluminium in quite large amounts, plus (usually) a minor amount of yttrium or other rare earth element. In weight percentage terms, such alloys may be broadly defined as having the following compositions:
Cr - 10 to 50% PA1 Al - 4 to 30% PA1 Y - 0 to 3% PA1 Cr - 25% PA1 Al - 12% PA1 Y - 0.5% PA1 Co - balance PA1 Cr-16%, Al-3.4%, Ti-3.4%, Co-8.5%, W-2.6%, PA1 Mo-1.75%, Ta-1.75%, Cb-0.9%, Fe-0.5%, Si-0.3%, PA1 Mn-0.2%, C-0.17%, Zr-0.1%, B-0.01%, Ni-Balance. PA1 a) vapour deposited MCrAlY coating; PA1 b) further vapour deposited coating of aluminium on top of the MCrAlY coating; PA1 c) heat treatment of the duplex coated component to cause the inter-diffusion of the aluminium and the MCrAlY coatings. PA1 a) vapour deposited MCrAl(Y)-type first coating; PA1 b) application of further coating containing aluminium and at least one of the elements Hf and Pt--specifically, Al-Hf coatings were applied by a pack coating process and an alternative Al-Hf-Pt coating was applied by sputtering of Pt onto the first coating, followed by application of an Al-Hf coating by a pack coating process; PA1 c) heat treatment of the coated component to cause the inter-diffusion of the coatings, so producing a graded coating in which the outer portion consists of 10-30 wt. % Al and 2-5 wt. % Hf, with, in the case of the Pt-containing coating, 20-40 wt. % Pt.
Fe/Co/Ni - balance.
For example, one well known alloy, used as the basis of the present work, has a nominal composition in weight percent of:
As mentioned above, these alloys are frequently used as protective coatings for superalloy components exposed to high temperatures in corrosive environments. In tests, we have found that the above CoCrAlY alloy, in the form of an overlay coating applied by an argon shrouded plasma spraying technique, has given satisfactory high temperature protection and mechanical properties for nickel based superalloys such as IN738. This superalloy is available from the International Nickel company, and has the nominal composition in weight percent of:
One problem with all protective coatings of this type, particularly in highly corrosive marine environments, is gradual degradation in service due to continued mobility of elements within the coatings and from the base material at the high operating temperatures of the turbines of gas turbine engines. Elements such as Mo,W,Ti,Ta, and Cr present within a superalloy such as IN738 migrate into the coating, thus reducing the basic mechanical properties of the superalloy. Furthermore, elements at the surface of the coatings, such as Al,Cr,Co,Ni,Ti, etc., are consumed during service by oxidation and sulphidation and therefore have to be replaced continuously to afford continued protection. Such replacement occurs by means of migration to the surface from the bulk of the coating and ultimately from the superalloy base material.
It is already known to improve the ability of MCrAlY protective coatings to resist high temperature oxidation and corrosion by aluminising them to produce an aluminide layer on top of the MCrAlY coating. In this context, "high temperature" means above about 750.degree. C. For example, British patent GB 1457033 discloses nickel- or cobalt-based superalloy components coated as follows:
Such coatings have been further improved upon by incorporating further refractory oxidation/corrosion resistant metallic elements such as platinum and hafnium into the outer aluminium coating and then diffusion heat treating the resulting duplex coating to produce an aluminide layer containing the refractory elements. For example, U.S. Pat. No. 4,123,594 discloses Fe,Co, and Ni based superalloy components which are coated as follows:
This patent also mentions rhenium and palladium as possible constituents of the outer portions of the coatings as additions to, or substitutions for, platinum and hafnium.
The contents of the above prior patent specifications are hereby incorporated by reference and should be consulted for further and more complete details of the compositions and processes summarized above.
It should be noted that, at least to some extent, MCrAlY coatings do act as a barrier between the base material and any outer aluminide coating to limit migration of base material elements, particularly aluminium and chromium, during service.
We prefer to apply MCrAlY coatings by the argon shrouded plasma spray process as mentioned above, proprietary to Union Carbide Coatings Service Corporation, of Indianapolis, U.S.A. After spraying, the coatings can be aluminised, but must, of course, be heat treated to inter-diffuse them with the base material.
As a result of aluminising an existing MCrAlY coating, stable aluminides of the "M" constituent(s) of the MCrAlY coating are produced in the top layer of the resulting coating, such as nickel and/or cobalt aluminides, resulting in improved hot oxidation/corrosion resistance. In service, some of the Al component in the aluminides gradually migrates to the surface of, the coating, where it is converted to alumina (Al.sub.2 O.sub.3), so producing a corrosion/oxidation resistant barrier. Nevertheless, this barrier is gradually eroded and is replaced by further oxidation of aluminium migrating from the aluminides below.
Ideally, the required extra Al to maintain the concentration of aluminides in the surface layers of the coating could be supplied from the MCrAlY inner portion of the coating. But in the case of MCrAlY alloys with aluminium concentrations near the lower end of the above-mentioned ranges (say, 4 to 20 wt. %, e.g., 12 wt. %, as in the specific example mentioned above), the Al content is insufficient for the migration mechanism to be able to continue to supply the deficient surface layers with Al for a lengthy period. Despite this, MCrAlY coatings with such relatively low aluminium contents are preferred because higher Al contents tend to make the coating brittle in service after the necessary diffusion heat treatment has been carried out.
Consequently, it is desired to improve the hot corrosion and oxidation performance of aluminised plasma-sprayed MCrAlY coatings in which the MCrAlY component of the coating has a relatively low Al content of 4 to 20 wt. % in the as-applied condition.
In addition to the hot oxidation/corrosion resistance referred to above, it is also desirable to improve the ability of MCrAlY coatings to resist low temperature oxidation and corrosion, "low temperature" in the present context meaning about 550.degree.-750.degree. C. This, of course, should be accomplished without any detrimental effect on the high temperature protection afforded by the coatings.
One known way of improving low temperature oxidation/corrosion resistance of any coating system is to increase the concentration of chromium, at least near the surface layer of the coating. Generally speaking, it has been found that low temperature oxidation/corrosion resistance improves as Cr concentration levels in the coating increase to about 40 wt. %, but that further increases above 40 wt. % are deleterious. The process of increasing the Cr content of an existing coating is called chrimizing, and is usually accomplished by known vapour or pack chrimizing methods, such as those marketed by Chromalloy Corporation.
Chromising improves low temperature oxidation/corrosion resistance of coating systems because Cr oxidises to form chromia, Cr.sub.2 O.sub.3, which forms a protective oxide film at the coating surface. Unfortunately, chrimizing is ineffective to improve high temperature oxidation/corrosion resistance of coating systems. This is because Cr.sub.2 O.sub.3 dissociates into Cr and gaseous CrO.sub.3 at temperatures above about 750.degree. C.